1. Field of the Invention
This invention relates to sundials and more particularly, to a sundial which is capable of indicating both the time of day and the approximate day of the month responsive to the casting of a shadow by spherical gnomons on a pair of semi-cylindrical dials positioned at a selected angle with respect to the axis of the earth. Ideally, the sundial is fitted with a pair of dials which are angularly and pivotally-disposed with respect to the base, in order to effect traversal of the sun's shadow along multiple planes on the concave surface of the dials.
The basic problem with conventional sundials is that they do not indicate clock time. Conventional sundials use a solid gnomon which typically casts a shadow on a flat plate located on or positioned parallel to the surface of the earth. This mechanical arrangement is useless as a time indicator in the early and late portion of the day, since due to the length of the shadow cast, the shadow is barely readable in the middle of the day. Furthermore, these sundials provide no correction for the changing position of the sun relative to clock time. A correction for the position of the sun as it relates to clock time is indicated on many globes as an analemma, shown as a "FIG. 8" in the torrid zone on these globes. The analemma indicates the advance or retardation of the sun relative to clock time, as well as the latitudinal position of the sun during the changing seasons. This time dislocation results from the earth following an elliptical, rather than a circular path about the sun, with the sun positioned at a foci of the ellipse. The elliptical path of the earth may be divided into four parts; a first long path which extends from the equinox position to the farthest apogee, or the summer solstice; a second long path which extends from the farthest apogee back to the opposite equinox position; a first short path which extends from the sun to the near apogee, or the winter solstice; and a second short path from the the near apogee back to the starting equinox position. During the first long path, the earth rotates such that with each passing day, it must rotate a fractional degree greater than 360 degrees, in order to bring the same point on earth in line with the sun. Accordingly, the sun "falls behind" relative to the clock. This lag accumulates and reaches a maximum with the solstice, at which point the earth returns along the second long path. However, when the earth reaches this point in its path around the sun, the rotation of the earth is such that a fractional rotation less than 360 degrees is required to place the same point in line with the sun and the sun begins to advance with respect to local, or clock time. This phenomena is repeated in the two short paths, respectively, and the effect of this phenomenon is to create four different regimes with respect to the advance and retardation of the sun relative to our clock. Close examination of the analemma on a globe reveals that the FIG. 8 is asymmetrical in each of its four parts. Thus, the difficulty in dealing with a sundial correction for this effect has heretofore prevented the construction of a sundial which indicates the correct time regardless of the date.
Another difficulty with conventional sundials is inapplicability of these devices to any but the middle latitudes of the earth. When one considers, for example, that the sun may rise as early as 4:30 a.m. in Boston, Mass., it is understood that state-of-the-art sundials are not suited to the indication of time at such hours, nor are they designed to operate at the late settings of the sun which occur at high latitudes. It was desired to create in the current invention a sundial which will register the correct time, regardless of the season or day of the year and regardless of the latitude of the sundial position.
The sundial of this invention consists of two semi-cylinders, or dials, 180 degrees each in angular length and 47 degrees in angular width, as measured from the midpoint of the center plane or diameter of each dial. This measuring point coincides with the position of the shadow device, or point gnomon, which is itself a small sphere having a diameter sufficiently large to cast a detectable shadow. The dials are oriented such that the plane of the sun's path coincides with the plane of the center of the dials at the two equinoxes and one dial is set facing east and the companion dial facing west, with each dial capable of indicating twelve hours of the sun's passage. The longitudinal axes of the dials are ideally located so as to be parallel with the axis of the earth. Accordingly, when the sundial is located at a point on the equator, the longitudinal axes of the dials are parallel to the earth's surface and when the sundial is located at the poles, the longitudinal axis is perpendicular to the earth's surface. In general, the angle of the longitudinal axis of the dials with respect to the earth's surface is equal to the angle of the latitude of placement.
When the sundial of this invention is so oriented, the shadow of the sun at the spring equinox will trace a pattern along the half-circumference, or the length of the dials exactly in the center of the dials. As summer approaches, the shadow will fall to a plane along the lower segment of the half-circumference of the dials as the sun rises higher from the horizon, until the summer solstice occurs on June 21, to a maximum angular displacement from the center circumferential plane of the dials of slightly more than 23 degrees. The sun will then trace consecutive, parallel paths across the concave dial surfaces back toward the center circumferential plane of the dials until the fall equinox, at which time the daily shadow traces will rise on the dials with the sun lowering on the horizon. This continues until the winter solstice, which occurs on December 21, for a maximum deviation of slightly more than 23 degrees from the center circumferential plane of the dials. With appropriate markings located on the dials, whether these markings are characterized by straight lines extending around the circumference of the dials or by more fanciful markings, one can closely estimate the day of the year from the shadow trace of the gnomons, depending upon the scale of the sundial and presuming the ability to detect the difference between the seasons.
A 24-hour day is based on the apparent passage of the sun through 24 angular periods of 15 degrees, each for a total of 360 degrees, or one complete rotation of the earth on its axis. The semi-cylinders, or dials in the sundial of this invention are thus marked with lines in a transverse direction to the circumferential markings for the day of the year, each 15 angular degrees apart for each hour. The shadow cast by the point gnomon on each dial is therefore capable of indicating twelve hours of time and with two dials opposing each other, all 24 hours of the clock can be indicated by the sundial. However, since the entire 24 hours of the clock are usefuly only to an inhabitant of the north or south poles for a short period of time each year, then for practical purposes, the sundial of this invention can be made responsive to the sun for a shorter period of time, for example, between the hours of 2:00 a.m. and 10:00 p.m. However, it will be appreciated that a selected scaling of the dials to cover a desired period of time can be effected, according to the desires of the designer.
Since the sundial of this invention is capable of indicating approximately correct time, regardless of the latitude of placement, throughout a full 24-hour day if desired, an adjustment for correcting the analemma, or the advance or retardation of the sun relative to clock time can be implemented to increase accuracy. With the dials correctly oriented, they may be attached to a rigid or portable framework which is mounted or positioned on an axis that exactly parallels the axis of the earth. This mounting is designed to facilitate rotation of the dials through an angle of approximately four degrees each way, or eight degrees total, to compensate for the approximate plus or minus sixteen clock minutes by which the sun is ahead of or behind the clock.
2. Description of the Prior Art
Various types of sundials are known to those skilled in the art. U.S. Pat. No. 4,355,470, dated Oct. 26, 1982, to Timothy E. Doyle, details an "Equatorial Sundial". The sundial of this invention includes a dial surface and an elongated, rod-like gnomon fixedly secured in operative relationship thereto. The dial surface is generally semi-cylindrical and concave toward the sun and the circumferential portion of the surface is formed in the shape of a partial helix. The gnomon lies on the axis of the dial surface, such that the longitudinal extent of the surface is transverse of the gnomon at an obtuse angle thereto. In a given geographical location, the sundial is so oriented relative to the earth that the gnomon is parallel to the earth's axis of rotation. With a proper pitch of the helical form of the dial surface, the opposite end portions thereof are offset in a direction parallel to the axis of the gnomon, so that neither shade the dial surface, either in the morning or evening. U.S. Pat. No. 4,135,357, dated Jan. 23, 1979, to George Ashton, entitled "Solar Chronometer", details a sundial which is mounted in alignment with the celestial pole and an equatorial member, preferably in the shape of a ring. The solar chronomotor is situated with a style aligned with a celestial pole and an equatorial member aligned with the equator. The style and equatorial member cast shadows upon a chart, such that the point where the shadows intersect indicates the current hour and data. U.S. Pat. No. 4,081,911, entitled "Sundial" was issued on Apr. 4, 1978, to Albert M. Eldridge. The sundial detailed in this patent includes a body portion having a cylindrical wall. A gnomon is connected to the body portion for casting a shadow on the cylindrical wall and a pair of flexible sheets are alternately mountable on the cylindrical wall. Fasteners allow rotation of the sheets about the longitudinal axis of the cylindrical surface to correct for differences between local civil, or clock time and standard time of the time zone of observation. The time lines are configured in accordance with the equation of time and each sheet contains time lines which designate the time occurring in either the fall or spring solstice. A "Sun Compass" is disclosed in U.S. Pat. No. 4,028,813, dated June 14, 1977, to Alfred M. Eldridge. The sun compass includes a base having a transparent hemisphere, a body movable witihin the base and a plate including a dial face, movable upon the body. Actuating members are connected to the plate and body for selectively moving the plate relative to the body and the body relative to the hemisphere. The actuating members are accessible externally of the hemisphere and include rotatable knobs having scales associated therewith, for indicating the extent of movement of the plate and body. A transparent hemispherical top is mounted over the base hemisphere. U.S. Pat. No. 4,102,054, dated July 25, 1978, entitled "Sundial", to Kemp S. Lewis, discloses a sundial which is adjustable with respect to its mounting location to correct for the earth location in both longitude and latitude. The sundial includes solar time equation indicia and an indicator to correct real solar time to mean solar time and incorporates other indicia to determine the calendar date based on sun declination.
It is an object of this invention to provide a new and improved sundial which is capable of indicating both the time of day and the approximate day of the year.
Another object of the invention is to provide a sundial which is capable of correcting for the advance and retardation of the sun relative to local, or clock time, in order to provide an accurate indication of clock time throughout a 24-hour time span.
Still another object of this invention is to provide a new and improved sundial which is provided with a fixed base and a hingedly-mounted time-indicated portion for accurately indicating the time of day in a time span of up to 24 hours at any latitude of placement and at both equinoxes and solstices.
Yet another object of this invention is to provide a new and improved sundial which can be utilized at any latitude on the earth and is capable of indicating the correct time as closely as one may interpolate the position of the shadow of a point gnomon between the nearest hour or minute marks on a scale.
Yet another object of the invention is to provide a new and improved sundial which is characterized by a fixed base portion, a pair of semicylinders or dials pivotally mounted to an adjustable frame, which frame is hingedly mounted to the base portion, in order to facilitate a pair of curved, time-indicating surfaces which are not distorted by curvature and present an effective surface for reading a shadow cast by spherical point gnomons suspended from the curvature of the dials, respectively.
A still further object of this invention is to provide a new and improved, portable or permanently mounted sundial which is characterized by a base that adjustably mounts a pair of curved, time-indicating dials which are provided with spherical gnomons suspended along a diameter in the center thereof, for sequentially casting a shadow on the dials and accurately indicating the time of day and the approximate day of the year for any time period, including a full 24 hour day.